In general, a thermal power plant generating power by using coal as fuel combusts coal of approximately 180 ton/hr per 500 MW and supplies coal equivalent to approximately 37 ton per one pulverizer to a boiler. In the 500 MW thermal power plant using the coal, approximately 6 coal storages having a capacity of approximately 500 ton are installed and in 5 coal storages, the coal is normally supplied and one remaining coal storage is operated as a coal yard capable of reserving coal which may be preliminarily used during a predetermined period.
Moreover, in the thermal power plant using the coal as the fuel, a standard thermal power design criterion for the coal is designed to use low-moisture bituminous coal of 6,080 Kcal/Kg and 10% or less. In some thermal power plant, imported coal is used and some bituminous coal among the imported coal average moisture content of 17% or more may be provided to decrease combustion efficiency of the boiler. When a calorific value of coal using 5,400 Kcal/Kg as a standard thermal power combustion limit is low, it is anticipated that a power generation amount decreases and fuel consumption increases due to the decrease in combustion efficiency. Moreover, when subbituminous coal which is high-moisture low calorific coal is used, a moisture content is higher than a design criterion, and as a result, a transport system transporting the coal is not smooth and when the coal is pulverized by the pulverizer, efficiency decreases and the combustion efficiency decreases due to partial incomplete combustion, and unbalance of heat distribution, which occurs in the boiler and the boiler may also operate in an abnormal state. However, in the thermal power plant, a use weight of the subbituminous coal gradually increases up to approximately 41 to 60% in order to reduce fuel cost.
Further, preference to the thermal power plant increases due to anticipations of a global business recovery and confronting a safety problem due to breakage of a nuclear power plant by a big earthquake of Japan, and as a result, it is expected that the demand and cost of the coal will continuously increase. As an environment of a global coal market is changed from a consumer to a supplier, stable supply and demand of the coal are actually difficult and a yield of the high caloric coal is prospected to be maintained at a current level, and as a result, the unbalance of the supply and demand of the coal is anticipated.
The low caloric coal in total global coal deposits is approximately 47% and the deposits are large, but the caloric value is small and the moisture content is high, and as a result, it is difficult to completely combust the high-moisture low caloric coal due to a combustion failure, and the like during combustion. Therefore, the high-moisture low caloric coal is disregarded on the market. Globally, a tendency to depend on a stable price of petroleum and low-priced production cost of the nuclear power generation has been high in recent years, but construction of a lot of thermal power plants using the coal has been planned due to a rapid increase of a petroleum price and anxiety about the nuclear power generation in recent years.
As a technique (thermal drying) that dries the coal in the related art, a rotary drying method that dries coal particles in a cylindrical shell with high-temperature gas while rotating the cylindrical shell into which the coal is input, a flash (pneumatic) drying method that dries the coal by lifting the high-temperature dry gas from the bottom to the top while supplying the coal from the top to the bottom, and a fluid-bed drying method that dries the coal while the high-temperature dry gas is lifted up while accompanying minute particles are primarily used.
The coal is divided into surface moisture attached to a porosity between the coal particles and coupling moisture coupled to air holes in the coal. As the surface moisture, most moisture sprayed during a washing process, and transport and storage in a district and the amount of the surface moisture is determined according to a surface area and absorptiveness and as the particles are smaller, the surface area increases and a capillary is formed between the particles to contain the moisture, thereby increasing the moisture content. The coupling moisture is formed at a generation time of the coal and the amount of the coupling moisture is the smaller in the order of brown coal, soft coal (bituminous coal and subbituminous coal), and anthracite coal. When the coal has much moisture, the caloric value decreases and transport cost increases, and as a result, controlling the moisture is required during processes such as mixture, pulverization, separation, and the like of the coal.
Moreover, in a multi-stage dryer, that is, a device that dries the coal by spaying high-temperature reheat stream below a dryer while transporting the pulverized coal through a conveyor with a plurality of through-holes through which reheat steam passes or a plurality of coupled transport plates, even though the reheat is dispersed while the injected coal is not evenly dispersed, the moisture included in the coal cannot be effectively dried. As a result, the number of stages and the length of the dryer for drying the coal needs to be increased and a supply amount of the reheat steam for drying increases, cost and time required for drying the coal increase.
Further, when pulverized coal is supplied to the dryer in order to dry the coal, coal particles are scattered and the dust is thus generated. The dust acts as environmental contamination or causes malfunction of a coal drying system. Further, frequency cleaning depending on maintenance of the coal drying system is also accompanied.
As the prior art related with the present invention, in Korean Patent Registration No. 10-0960793, in a low-grade coal stabilization device, a wave-type oscillating flow plate for uniformly mixing primarily dried low-grade coal with heavy oil powder input for increasing dry efficiency is provided. The oscillating flow plate may evenly mix the low-grade coal and the heavy oil powder and there is a problem in that dried steam for drying the coal is not uniformly injected on the coal surface and thus dry efficiency may be decreased.